Leveling support



Dec. 27, 1949 Filed Sept. 24, 1942 P. F. SHIVERS LEVELING SUPPORT 5 She'e'ts Sheet 1 INVENTOR. P/i't/L -i 5/11 VE/75 w de/V WL Dec. 27, 1949 P. F. SHIVERS 2,492,244

LEVELING SUPPORT Filed Sept. 24, 1942 5 Sheets-Sheet 2 INVENTOR.

P/IUL fi 511 555 BY Za A. W

Dec. 27, 1949 P. F. SHIVERS LEVELING SUPPORT 5 Sheets-Sheet 3 Filed Sept. 24, 1942 INVENTOR Pf/UL It 511/ VE/RS P. F. SHIVERS LEVELING SUPPORT Dec. 27, 1949 5 Sheets-Sheet 4 Filed Sept. 24, 1942 B'nventor Dec. 27, 1949 P. F. SHIVERS LEVELING SUPPORT 5 Sheets-Sheet 5 Filed Sept. 24, 1942 \\\M\&

Patented Dec. 27, 1949 LEVELING SUPPORT Paul F. Shivers, Edina, Minn., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn, a corporation of Delaware Application September 24, 1942, Serial No. 459,517

'7 Claims.

The present invention is directed to levelling supports and particularly to an automatically operated aerial camera levelling device.

An object of the invention is to provide an improved levelling device including a gimbal supporting a track on which the device being maintained level may be rotated about a vertical axis.

Another object of the invention is to provide an improved levelling device which includes a support rotatable on a track which is maintained automatically in level position.

An object of the invention is to provide an improved levelling device especially adapted for use with aerial cameras which will automatically maintain the camera in level position even though the airplane in which the camera is mounted is subject to various inclinations due to uneven air conditions.

Another object of the invention is to provide an improved automatically controlled levelling device including a gimbal support in which the control motors carried by the gimbal are so disposed as to substantially counterbalance one another.

A further object of the invention is to provide an improved gravity responsive pick-up unit for controlling an electric motor to correct for any inclination of the device being maintained in level position.

A further object of the invention is to provide an improved gravity influenced control device in which a pendulum controls the impedance of an electric circuit.

A still further object of the invention is to provide a gravity influenced control means which is substantially unaffected by accelerations thereof.

Other objects will be apparent from the following detailed description, from the claims, and from the drawings, in which:

Figure l is a plan view of an aerial camera levelling device incorporating certain features of my invention,

Figure 2 is an elevation, partly in section, of the device shown in Figure l, the section being taken substantially on line 22 of Figure 1,

Figure 3 is a continuation of the view shown in Figure 2,

Figure 4 is an elevation of the device shown in Figure l, with certain portions of the left-hand side broken away,

"Figure 5 is a schematic wiring diagram of an electric control circuit for one of the motors used in my invention,

Figure 6 is an elevation of one of the gravity influenced pick-up means employed in the camera levelling device of Figures 1 through 5, together with a schematic diagram of the electric circuit in which it is employed,

Figure '7 is a plan view, partly in section, of the pick-up means shown in Figure 6,

Figure 8 is a plan View of an alternate form of gravity influenced pick-up means, and

Figure 9 is a sectional elevation taken substantially on line 99 of Figure 8.

Referring now to the drawings, a levelling device which may be used, for example, to support an aerial camera, is shown in Figures 1, 2, 3 and i. A generally circular frame It has four mounting lugs ll integral therewith, the lugs H being adapted to mount the frame 10 in a normally horizontal position on the interior of an airplane. It is intended that the frame Ill be disposed above an opening in the bottom of the airplane fuselage. Suitable means are of course provided for carrying the frame In within the airplane. A gimbal ring I4 is pivoted in the frame In on trunnions l5 and I6, so that the gimbal ring 14 pivots on a normally horizontal axis. The gimbal ring l4 carries an annular frame member l8 upon a pair of trunnions l9 and 2B, which are disposed so that the member I8 is pivoted on a normally horizontal axis at right angles to the axis through the trunnions l5 and It. The two sets of trunnions l5, l6, and I9, 20 provide a support which permits the member [8 to be maintained in a level position regardless of any inclination of the frame 10, within reasonable limits.

A plurality of rollers 23 (see Figs. 2 and 3) are mounted on stub shafts carried by the annular frame member 18 and projecting inwardly therefrom. The rollers 23' are disposed in a single plane and are provided with a V-shaped periphery so as to engage a correspondingly V-shaped groove '25 in the under side of a flange which is integral with another annular frame member 26. The annular frame member 26 is supported by the rollers 23 in the member l8, so as to be rotatable about an axis normally vertical with respect to the member IS. A pair of camera supports 30 are secured to the member 26 by means of studs 3|. Sponge rubber pads 33, and sponge rubber washers 34 are used to space the supports 30 from the frame member 26 and the studs 3| respectively, so that vibration occurring in the member 26 will not be transmitted to the camera supports 30. Each of the camera supports 39 carries a pillow block 29, on which is pivotally mounted a clamping lever 36. Each of the levers 36 is apertured to receive a threaded bolt provided with a thumb screw 31 above the lever 36. The construction is such that a projection on the camera or other supported device may be firmly clamped between the lever and the pillow block 29. Each of the blocks '29 has a pin 38 extending into the circular opening therethrough which aids in preventing rotation of the camera with respect to the camera support all. A camera 39 is represented by dotted lines in Figures 1 and 4.

One of the camera supports 30 carries a controlling pick-u-p 40 which will be described in detail in connection with Figures 6 and 7. The pick-up 40 is of the gravity influenced type and since it is mounted on one of the camera supports it is sensitive to any deviations of the camera from level position.

In order to compensate for lateral drift of the airplane and to position the camera about a vertical axis so that it will be in line with the true direction of flight 0f the airplane, a motor 44, hereinafter referred, to as the azimuth motor, is provided to rotate the frame member 26 on the rollers 23. The motor 44 is carried by the frame member 26, and drives a pinion 45 through a reduction gear 46. The pinion 45 engages a rack 48: on the lower periphery of the member 18. The motor is. of the reversible type and may be controlled by the pilot of the airplane. The control circuit for the motor 44 will be described in connection with Figure 5. A follow-up mechanism 49: is mounted integrally with the motor 44 and this follow-up mechanism will also be described in connection with Figure 5.

' The member I8 is provided with a downwardly extending portion 5i (see Figs 3 and 4) carrying a reversible electric motor 52, hereinafter referred to as the rollmotor, which drives a pinion 54 through a reduction gear 55. The pinion 54 engages a gearsegment 51 which is integral with the gimbal ring I4 and extends downwardly therefrom. Operation of the motor 52 and rotation of the pinion 54 causes relative rotation of the member l8 and the gimbal ring l4 on the trunnions l9 and 20. It will be noted that the motor 52 and the azimuth motor 44 are normally diametrically opposed to each other with respect to the vertical axis of the mechanism. The motors therefore counterbalance each other, and an eccentric load which might tend to rotate gimbal M. in the trunni-ons l5 and I6 is thereby avoided. The motor 52 is controlled by the pickup 40,, the electrical connections therebetween being diagrammatic-ally illustrated in Figure 6.

Afreversible electric motor 66 (Figs 1, 3 and 4), hereinafter referred to as the pitch motor, is carried by the frame iii. The motor 58 operates. a pinion'fil through a reduction gear 62 and the pinion 51 cooperates with a gear segment 64 which, is. formed integrally with the gimbal ring 14. The motor therefore serves to ang-ularly position the gimbal ring [4 in the frame Ill about the axis |5 l6,. The, pitch motor 60, like the roll motor 5Z,is. controlled by the pick-up 40.

Since the gimbal ring I 4 is pivoted in the frame L0 onthe: axis |-5|6 and the member I 8 is pivoted in the gimbal ring l4 on the axis |9-26 which is, at right angles to the axis I5 l6, it will be seen that the. motors 52 and Bi] may be positioned to compensate for any inclination of the frame l0 within reasonable, limits.

All, electrical connections have been omitted from Figures 1 through 4 since it was felt that the disclosurev would be confused rather than clarified by showing such electrical connections.

4 They are shown diagrammatically in Figures 5 and 6.

Referring now to the wiring diagram of Figure 5 directed to the azimuth control, it will be seen that the motor 44 which is of the split-phase induction type includes a pair of windings 66 and 67, which are spaced ninety electrical degrees apart, in accordance with the conventional construction of such motors. The motor 44 also includes an armature 68 which operates the pinion 45 through a reduction gear box 46. A condenser 69 is connected in parallel with the winding 61. The common terminal of the windings 66 and 61 is grounded at ID.

The capacitance of the condenser 69 and the inductance of winding 51 are so proportioned that the two form a series resonant loop circuit. This loop circuit is supplied with energy by transformer action from the winding 66, but the current fiow in winding 61 caused by this transformer action is not sufiicient, or of the proper phase relationship, to cause rotation of motor armature 68. When a slight additional amount of energy is supplied to winding 61, however, it

is energized sufficiently to start rotation of the armature 63 in a direction determined by the phase of such additional amount of energy.

Power is supplied to the motor 44 from a transformer 72 having a primary winding '13 which is connected to a suitable source of alternatingcurrent. The transformer 12, has, a secondary wind.- ing 75, one terminal of which is grounded at I6 and the other terminal of which is connected to the motor winding 66 through wire 71, a condenser i8, and wire 19. The winding 661s therefore constantly energized through the condenser 18.

The transformer 12 also includes a. pair of secondary windings 82 and 83 which in conjunction with an amplifier generally illustrated at 85 supply power to the winding; 621, of the motor 44. The upper end of the secondary winding 8-2- is connected to an output terminal 85 through a wire 81, while the lower end of the secondary winding 83 is connected to the output terminal 88 of the amplifier 85 through a wire 89. The mid-point of secondary wind-ings B2-83 is connected to motor winding 61 through a wire 90. The amplifier 85 may preferably be of the type disclosed in Figure l of the co-pendin application of Albert P. Upton, Serial No. 431,561, dated April 3, 1942, now matured into Patent No. 2,423,534, issued July 8, 1947. Power is supplied for operation of the amplifier 85 from a transformer 9i having a primary winding 9-2 connected to a suitable source of alternating current and a secondary 93 which is connected to the amplifier. The amplifier 85 also. includes input terminals 95 and 96, the, input terminal 96 being grounded at B1. The overall, effect of amplifier 85 is to supply the motor winding 61 with alternatin-g current of a given phase or the opposite phase, depending upon the phase.- relation between an alternating signal impressed on the input terminals 95 and 9-6 and that or the alternating current existing in the secondary windings 82 and 83 of the transformer 12. It may be considered that current supplied to the winding 6-! of themotor 44 will be either in phase with the current flowing in the secondary winding 15 of the transformer 12 or degrees out of phase with that current. Since the motor 66 is supplied with current from the transformer 12 through the condenser 18', the current flowing in that winding is substantially ninety degrees out of phase with. the-current flowing in :the primary winding .13 .of the transformer :12. .Hence, ;the armature 5.8 eofcthe motor :44 rotates v in opposite directions, depending uponthephase of the alternatingsignal =impressed enainputgterminals 9.5 and 96.

'The phase of the .alternating rsignal supplied to the amplifier 285 is :determined :by a :bridge circuit which includes an electrical resistance I and asecondselectricalresistance 'IOI connected in parallel. I-he -two ends of these resistances are interconnected by iwires I02 and A transformer I Oil-having a primarywinding IBIi-anda secondaryewinding I0'Iserves as a source of electrical'poteritial for theabridge circuit, the upper :end of the secondary winding I01 beingconnectedctolthe wire I02 :by a wire I09 and the lower endof the secondary winding being connected to the winding I03 (by .a wire I I-0. -Althoughthetransformers"l2and I05 have been shown separate, in actual practice the secondary I 51 of i. the transformer I05 would be anadditional secondarywinding on the transformer-I2. If the transformers are separate as shown, the primary '13 of the transformer 12 and the primary I06 of the transformer- I 05 should be ener-gizediromthesame source of-alternating current,-so that the :phaseof theelectrical potentialsin the var-ious'secondary windings would bear a-defin-ite relation'toone another. A slider I I2 cooperating with the resistanee l00 is connected to the'input terminal -95 of the amplifier '85 'by a wire I I3. A sliderd I5 oooper-ating'with the resistance IOI isconnectedto theinput terminal 96-of the amplifier 85 by-a wire H6; slider H5 together with the r-esistance IOI constitute the--f011OW-up =mechanism 49- illustrated in Figures 1 and 2, the slider H5 being positioned bythe armature'BB ofthe-motor44 through a reduction gear. The arrangement is such that the slider II 5- has -a A definite position on the resistance IfiI for every-position-of=the pinion 45 on the-rack 48. "Th -sliderl I2 is-positioned on the resistance Hi0by-means-ofaknob- I I8 which also positions a pointer II9 which -eooperates with suitable mama-12a. Theresistan'ce I00 together with the slider -II2, --knob -II8, pointer -I I9 and'in'dicia I"20=may-suitably be -located'in the pilot's compartment of =the-airplane. The airplane pilot may. then position :the pointer *1 I 9 at asuitable point on indicia I 2-0 corresponding to the'number of degrees of drift-oftheairplane.

When theslider 2 is located ata position on the resistance lflfl -so-thatit is at-the same potential as the slider l-I5-on theresistance- IOI,

no input 'voltage will be supplied to "the terminals 85 and 96 of the amplifier 85. When this conditionexiststhesecondary windings 82 and 83 supply no current to' thewinding 61 of the motor-44 and-the armature=58is at rest.

If-the-upper end of the secondary I'0-I of the transformer 105 be assumed to be at zero potential, then the left-hand ends of resistances 100 and IN are also at zero potential. It the slider H2 is moved toward the-left ,on the resistance WEI the potential applied ;to the input terminal 95 of the amplifier ;85 willbe reduced, and since the slider I I5 cooperating with the resistance- IOI *has not'yetmovedythe input terminal 95 of theamplifier 85 which is connected to theslider H5 willbe at a higher potential than the input terminal 195. "Therefore, an alternating potential Willbe applied .to the :input terminals of the amplifier 85. A current then "flows in .the-mot0rwinding51of such a phase The .with;:m$n 1t0t th sournt mowin wi din seas returnhe matur iii a dir ction to move :the slider I I 5-;towards;th slider I I5 i moved 7 1 1 11 let tance vto brin the -.notentia1 a h slider :I 1:5

sliders H2 and H15 and therefore between the two input terminals of'the amplifierc85. Again assumingthatthe upper end of thezsecondary i0? is at zeropotential and'thatLthe left-hand ends of the resistances I00 and JIUI rare also at zero potential, thenthe=slider 1H2 is at a higher potential than the slider I H15. An alternating potential is again applied to the input terminals of the amplifier but this signal=is degrees different in phase from the signal'applied when the slider H2 was'at-a lower potential than the slider I15. The'amplifier=85thenenergi-zes the windingt'i of the motor 44 so that the phase relationbetween the-current-flowing inthe motor windings 65 and!" is such as-"to rotate the armature 58 in the-direction to move the slider H5 toward the right. When the slider H5 is moved to the right-a distanee sufiicientto render the potential at the slider H 5 the same as at the slider H2, no signal is impressed on the input terminals of the amplifier 85 and the motor 44*nolonger-is operated.

It will be seen therefore that the motor :44 and therefore the frame member "26 referred to in conneotion -withFigures1through 4.,and also the camera itself assumes a definite position about the -norma11y verticalaxis with respect to the airplane and the frame I0 of the cameralevelling device for each position of the pointer H0 which cooperates with the indicia 20. I

The controlling pickehp .40 fonthemotorsBZ andiiil is shown in Figures- Sand ,7; Thepick-up 45 includes a pairof gravity influenced control units I25 andiZi; which are identioalandwhich are mounted within a oasingjIZS Whichis carried by one of the supports 30 so that its position corresponds to that of the camera or other .supported device. "The pick-up .nnit !2 :5.is in control of roll motor 52 while the pick-up unit I26 is in control of the pitch motor 160. Since-the pick-up units I25. and I 26 are identical; only. the

unit I25 willbe described in detail. .An angle member I29 carried within the casing I28 serves as a support for a backplate l30 for the pick-up unit 125. Theplate I3 0 ispivoted at its lower end in the angle member I29 ona screw I32, while the upper endof the plate .I30is provided with aslot I33 throughwhich ascrew I34-passes into .the angle member 129i 'The relative position between the plate [30 and the member 1.2.9

and therefore between plate I30 andsupport .30 may be adjusted byloosening the. screw 434 and selecting the proper position vforthe plate I30 before retightening that screw. An inverted U-shaped member I38 has one of its legs I39 secured to the plate I30. 'Ijh e .181; .139 of the U-shaped member L38 together with aniouter leg M5 support arod I42 pivoted vtherebetween. The i ner end o the rod l,.42.is pivot dint leg I3,9. at I43 twhilet e-onter. end of {the-rod I42 is pivoted in the leg I40 at I44. In order to pro"- vide an accurate and substantially frictionless support for the rod I42, the two pivots I43 and I44 are preferably of the jewel type. A pendulum I46 is secured to the rod I42 and therefore pivots with respect to the stationary parts of the device on the pivots I43-I44. The lower extremity I48 of the pendulum I46 is arcuate in shape and adapted to cooperate with a permanent magnet I49 which is secured to the plate I30. The arcuate portion I48 of the pendulum I46 also cooperates with soft iron member I50 which is also secured to the plate I30. The gap between the portion I48 of the pendulum and the permanent magnet I49 is very small as is the gap between the portion I48 and the soft iron member I58. The resulting effect of the structure just described is to provide magnetic damping for the pendulum I46. The pendulum I46 and its arcuate portion I48 are of some suitable material of low electrical resistance such as copper.

Also attached to the rod I42 and therefore movable by the pendulum I46 is an armature I52 which is adapted to cooperate with an E-shaped core member I53 which is secured to plate I30. The core member I53 has an upwardly extending center leg I54 and a pair of upwardly extending outer legs I56 and I51. The center I54 extends to a point on clos proximity to the central portion of the armature I52. The armature I52 is so shaped that as the pendulum I46 moves with respect to the stationary portions of the device, the air gaps between the armature I52 and the two outer legs I56 and I51 of the core I53 are varied inversely. A winding I60 surrounds the leg I56 of core I53 while a winding I6I surrounds the leg I51. If the pendulum I46 is moved in a clockwise direction as seen in Figure 6, the armature I52 would approach the leg I51 and recede from the leg I56, thereby increasing the impedance of the coil I6I and decreasing the impedance of the coil I 60. If the armature I 46 is moved in a counter-clockwise direction, the impedance of the coil I60 would increase and that of the coil I6I decrease. Of course, in actual operation the pendulum I46 will hang substantially stationary and it will be the remainder of the mechanism which is tilted.

The details of this variable impedance device, per se, are shown and claimed in the copending application of Willis H. Gille and Harold A. Petsch, Serial No. 453,914, dated August 6, 1942, now matured into Patent No. 2,371,236, issued March 13, 1945.

The operation of the pick-up unit I25 in conjunction with the roll motor 52 of the device as described in Figures 1 to 4 is as follows: The motor 52 includes a pair of windings I64 and I65 which drive an armature I66. The armature I66 drives the pinion 54 through the reduction gearing 55 and cooperates with the gear segment 51 to position the member I8 of Figures 1 to 4 with respect to the gimbal rin I4. The windings I64 and I65 of the motor 52 are spaced ninety electrical degrees apart in accordance with the conventional construction of split phase induction motors. A condenser I68 is connected in parallel with windin I64. The common terminal of the windings I64 and I65 is grounded at I69.

The capacitance of the condenser I68 and the inductance of winding I64 is so proportioned that the two form a series resonant loop circuit. This loop circuit is supplied with energy by transformer action from winding I 65, but the current flow in winding I64 caused by this transformer action is not suificient, or of proper phase relationship, to cause rotation of the armature I66. When a slight additional amount of energy is supplied to winding I84, however, it is energized sufliciently to start rotation of the armature I66 in a direction determined by the phase of such additional energy supplied to the winding A transformer I12 having a primary winding I13 is provided with a secondary winding I14, one end of which is grounded at I15. The other end of the Winding I14 is connected by a wire I11, a condenser I18, and a wire I19 to the end of the motor winding I65 which is not grounded. Since the winding I65 is supplied with current through the condenser I18, the phase of the alternating current in this winding is substantially ninety degrees out of phase with that in the secondary winding I14 of the transformer I12. The motor winding I64 when energized is supplied with alternating current from a secondary winding I8I or secondary winding I82 of the transformer I12. The junction between windings I8I and I82 is connected to the motor winding I64 by means of a wire I84. An amplifier I86 which is similar to that described in connection with Figure 5 and which is preferably of the type disclosed in Figure 1 of Patent No. 2,423,534, has an output terminal I81 which is connected to the secondary winding I8I of the transformer I12 by a wire I88, and an output terminal I90 which is connected to the secondary winding I82 by a wire I9 I. The amplifier I86 is supplied with power from a transformer I93 having a primary winding I94 and a secondary winding I95. The amplifier I86 is also provided with input terminals I91 and I98, the terminal I98 being grounded at I99. The amplifier I86, in effect, supplies winding I64 with alternating current of a given phase, or the opposite, depending upon the relation between the phase of the alternating signal impressed on the input terminals and the phase of the voltage existing in the secondary windings IBI and I 82 of the transformer I12. The direction of rotation of the armature I66 of the motor 52, as previously pointed out, depends upon the phase of the current supplied to the winding I64. Therefore, the direction of rotation of the armature I66 will depend upon the phase of the signal impressed upon the input terminals of the amplifier I86. When no signal is impressed on the input terminals of the amplifier I 86, no current is supplied to the motor winding I64 and the armature I66 of the motor 52 remains stationary.

The input terminals I91 and I 98 of the amplifier I86 are connected to a bridge circuit including the coils I60 and I6I of the pick-up unit I 25. The bridge circuit also includes a pair of equal resistances 20I and 202 which'are connected in parallel with the coils I60 and I6I. The input terminal I91 of the amplifier I86 is connected to the pick-up coil I60 by means of wires 204 and 205, and is connected to the pick-up coil I6I by means of wires 204 and 206. A wire 208 interconnects the Junction between the resistances 20I and 202 and the input terminal I98 of the amplifier I86. The pick-up coil I60 is connected to the resistance 20I by a wire 209, while the pick-up coil I6I is connected to the resistance 202 by a wire 2| 0. A transformer 2I2 having a primary winding 2I3 supplies power to the bridge circuit from its secondary 2 I4 which is connected to the resistance 20I by means of a wire 2I6 and to the resistance 202 by means of a wire 2". The primary 2I3 of the transformer 2I2'must be connected to the same source of alternating current as the primary I13 of the transformer I12 inorder that there be a definite relationship between the phase existing in the bridge circuit and that in the secondary windings of the transformer I12. Preferably, the secondary winding 2I4 of the transformer 2 I2 may be a supplementary secondary winding on the transformer I12.

The operation of the control circuit of Figure 6 is as follows: When the armature I52 is positioned by the pendulum I46 so that it is in midposition with respect to the legs I56 and I51 of the E-shaped core member I53, the inductance of the pick-up coils I60 and I6I is the same. As pointed out before, the junction between the coils I60 and NH is connected to the input terminal I91 of the amplifier I86. The junction between the two equal resistances 20I and 202 is connected to the input terminal I68 of the amplifier I86. An alternating potential is applied across the two resistances 20I and 202, and also across the two coils I60 and I6I. Since the inductances of the coils I60 and I6I are equal and since the resistances of the resistances 20I and 202 are equal, the junction between the coils I60 and I6l and the junction between the resistances 20I and 202 are at the same potential. Hence no alternating current signal is applied to the input terminals of the amplifier I86. As heretofore pointed out, when no signal is impressed on the amplifier I86, there can be no :energization of motor winding I64 and the armature I66 of the motor 52 remains at rest. This is the condition which exists when pick-up unit I25 is in its proper position with respect to the horizon. The member I8, as

illustrated in Figures 1 to 4, is also in its proper relation to gimbal ring I4 and no operation of the motor 52 is required.

In casethe'pendulum I46 and therefore the armature I52 is rotated in a counter-clockwise direction with respect to the core I 53, the inductance of the pick-up coil I60 is increased and that or the coil I6I decreased. With this condition the potential of the junction between the coils 'I60 and I6I and therefore the potential of the input terminal I91 .of the amplifier I86 is no longer at the same value as the junction between the resistances 20I and 202 and therefore of the input terminal I98 ofthe amplifier I86. Hence, an alternating signal is applied to the input terminals of the amplifier I86 and current flows in motor winding I 64 of .such a phase as to cause the armature I66 to rotate in a direction to move 'the gimbal I4 with respect to frame member I8 so as to return the'pendulum I46 and the arma ture I52 to their neutral position with respect to the core I53 which is fixed with respect to the camera support. When this condition is attained, a signal will no longer be impressed on the amplifier and the motor 52 will cease operation.

Likewise, if the pendulum I46 and the armature I52 are rotated in a clockwise direction with respect to the core I53, the inductance of the coil I6I is increased and that of the coil I66 decreased. The potential of the junction between the coils I60 and I6I is no longer equal to the potential at the junction between the resistances 20I and 202 and an alternating signal is impressed on the input terminals I91 and I96 of the amplifier I86. This alternating potential is, however,1 80 electrical degrees out of phase with the signal whichwas impressed on the amplifier when "the armaturel52was moved in a counter-clockwise-direction. This signal when applied to the amplifier I86 will result in a current flow in the winding I64 of the motor 52 of such a phase relationship with respect to the current flowing in the winding I65, that the armature I66 rotates in such a direction as to position the member I8 with respect to the gimbal ring I4 to re-level the pick-up device 40. Sufficient movement of the pick-up 40 will reposition the core I53 with respect to the armature I52 of the position shown in which the inductance of the two coils I60 and I6I is equal. When this condition exists there is no longer a signal impressed on the input terminals of amplifier I86 and the motor winding I64 is no longer energized and the armature I66 comes to rest. Although the operation of the device has been described as resulting from a movement of armature I52 and pendulum I46 about the pivots I43 and I44, it should be remembered that in the actual operation of the device the pendulum I46 and the armature I52 remain in a substantially fixed vertical position and it isthe movement of the pick-up unit I25 and core I56 with respect to the true vertical which causes actuation of the motor.

The pick-up unit I26 which is also mounted within the casing I28 of the pick-up 40 is, as heretofore pointed out, identical to the pick-up unit I25. The pendulum of the pick-up unit I 26 is pivoted on an axis at right angles to that of the pick-up unit I25, and it is therefore sensitive to inclinations of the camera levelling device in a direction at right angles to the direction in which the pick-up unit I25 is sensitive. It is not deemed necessary to describe the pick-up unit I26 in detail nor is it necessary to describe the control circuit by means of which the unit I26 controls the pitch motor 60 which is adapted to correct the position of the camera when the airplane in which the device is mounted becomes inclined in a fore and aft direction. The motor circuit and the amplifier and bridge circuits are identical to those described in connection with the pick-up unit I25 and the roll motor 52.

Referring now to the controlling pick-up shown in Figures 8 and 9, this device may be substituted for the controlling pick-up 40 which is described in detail in connection with Figures 6 and 7. This device includes a cup 225 which is adapted to be secured to one of-the camera supports 30. The cup 225 is screwthreaded internally at its upper or'open edge to receive a cover 221. The lower surface of the cover 221 is provided with a pair of grooves 228 disposed at right angles to each other. The grooves 228 are adapted to receive four U-shaped laminated iron core members 230, 23I, 232 and 233 disposed radially about the center of the cover 221 with each core member having its legs depending therefrom. Any suitable means such as clamps 234 may be provided to secure the core members in fixed position in the grooves 228 in the cover 221. The outer legs of the core members have coils 235, 236, 231 and 238, respectively, mounted thereon.

Opposite pairs of these coils are connected to cooperate in controlling the two levelling motors of the device. For example, the coils 236 and 238 correspondto the coils I60 and I6I of the pick-up I25 described in Figure 6, while the coils 235 and 231 correspond to the two coils of the pick-up unit I26 of the former device. A single iron armature 240 cooperates with all of the U- shaped core members 230, 23I, 232 and 233. The armature 240 comprises the upper surface of a float 242. The float 242 is provided with a sleeve 243 extending downwardly through its center.

which sleeve is screw-threaded internally to re ceive an adjustable bearing member 245 provided with a conical bearing surface at its upper end. A float pivot 246 comprising :a pointed rod is provided with screw threads at its upper end which cooperate with corresponding threads in an opening through the cover 221. The float pivot 24% extends downwardly from the center of the cover 22'! between the inner legs of the U-shaped core members. The lower end of the float pivot 2'45 is conical in shape and cooperates with the float bearing 245. The normal spacing between the core members and the armature 2553 may be adjusted by turning the float pivot 246 in the cover :22]. The entire space within the cup 22 around the float 242, the core members 230, 23 i, 232 and 233, the coils 235, 236, 231 and 238 is filled with a suitable liquid. A filling plug 248 is provided in the cover 221 for the purpose of filling this space with liquid.

The float pivot 2'45 pivots the float 242 at a point corresponding to the center of gravity of the float 242. This adjustment may be obtained by turning the bearing 245' in the sleeve 243. The float is so constructed that the center of gravity thereof is appreciably below the center of buoyancy. When the cup 2 is tilted invarious directions, the float will tiltto vary the distance between the armature 24 0 and the various core members to obtain the desired control effect since the float is pivoted at a point below its center oi buoyancy. Lateral accelerations of the device will, however, have substantially no effect on the position of the float 242 since it is pivoted at its center of gravity. Hence, acceleration of the airplane in which the device is mounted will have no appreciable effect upon the control of the level of the camera.

From the foregoing, it will be apparent thatI have provided a camera levelling device which is suitable for use in aerial photography and which is very sensitive and accurate in operation. Many modifications which embody the essential features of my invention will occur to those skilled in the art, and I therefore intend my invention to be limited only by the appended claims.

I claim as my invention:

1. In a levelling device, in combination, a support subject to inclinations from a normal position, a first member pivoted in said support on a normally horizontal first axis, a second member pivoted in said first member on a normally horizontal second axis at right angles to said first axis, a third member so carried by said second member as to pivot with respect thereto about a normally vertical axis, first reversible motor means carried by said second member at a point remote from said vertical axis and ada ted to cause rotation of said second member about said second axis, and second reversible motor means carried by said third member at a point remote from said vertical axis and normally diametrical- 1y opposed to said first motor means and adapted to cause rotation of said third member about said vertical axis.

2. In an aerial camera levelling device, in combination, a support subject to inclinations from a normal position, a first member pivoted in said support on a normally horizontal first axis, a second member pivoted in said first member on a normally horizontally second axis at right angles to said first axis, a camera carrying member so carried by said second member as to pivot with respect thereto about a normally vertical axis, first reversible motor means adapted to rotate said first member on said first axis,second reversible motor means adaptedto rotate said second member on said second axis, third reversible motor means adapted to rotate said camera carrying member about said vertical axis in both directions from a normal position, control means for each of said first and second motor means mounted on said camera carrying member, each of said control means being responsive to tilting and including a pair of electrical coils, magnetic circuit means for said coils including a core member and an armature having oppositely extending portions, each said portions forming a part =of the magnetic circuit for one of said coils, whereby :on movement of said armature the impedances of said coils are oppositely varied, a liquid filled container, and a float in said container associated with said armature for varying the position of said armature with respect to said core member when the control means tilted.

3. In a device of the class described, in combination: a support subject to inclination from a level attitude; a member to be maintained in a level attitude; motor means for adjusting the attitude of said member with respect to said support; a rigid float pivotal-1y responsive to departure of said member :from said level attitude; variable inductance means adjusted in accordance with the response of :said float; and means connecting said motor means in controlled relation to said impedance means, whereby to'cause correction in the attitude of said member whenever it deviates from the level.

4. In .a device of the class described, in combination: a support subject to inclination from a level attitude; .2. member to be maintained in a level attitude; motor means for adjusting the attitude of said member with respect to said support; a hollow float tiltable with respect to said member upon departure thereof from said level attitude; fluid .means buoyantly supporting said float in a totally submerged condition; variable impedance means adjusted in accordance with the tilt of said float; and means connecting said motor means in controlled relation to said impedance means, whereby to cause correction in the attitude of said member whenever it deviates from the level.

"5. In a device of the class described, in combination: a support subject to inclination from a level attitude; a member to be maintained in the level attitude; motor means for adjusting the attitude of said member with respect to said support; a float having its center of gravity lower than its center of buoyancy, and having a pivot located at its center of gravity; means mounted on said member and buoyantly supporting said float against said pivot, whereby to render said float responsive to tilt of said member; means varied in impedance in accordance with said response of said float; "and means connecting said motor means in controlled relation to said impedance means, whereby to cause correction in the attitude of said member whenever it deviates from the level.

6. In a device of the class described, in combination: a support subject to inclination from a. level attitude in any direction; a member to be maintained in a level attitude; motor means for adjusting the position of said member with respect to said support about each of a pair of normally horizontal axes; control means for said motor means comprising a float of magnetic material, .a fluid-containing housing pivotally mounting said float for relative tilting therebetween, impedance devices each comprising a magnetic core with a winding formed thereon; means supporting pairs of said cores in alignment with said axes, so that said float cooperates with said cores to oppositely vary the impedances of the coils of said pairs; and means energizing said motors, in accordance with the impedances of the coils of said pairs, to adjust the position of said member so that it assumes a level attitude.

'7. In a levelling device, in combination, a support subject to inclinations, a gimbal having a normal position and pivotally mounted on said support for movement about a predetermined axis, motor means for causing relative movement about said gimbal and said support, control means for said motor means comprising a rigid body, an inverted pivot carried by said gimbal, means buoyantly supporting said body against said pivot, with its center of buoyancy higher than its pivot point, for pivotal movement about at least an axis aligned with said predetermined axis, and means including variable impedance means responsive to pivotal movements of said body and acting as a result of inclination of said support to cause operation of said motor to restore said gimbal to its normal position.

PAUL F. SHIVERS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,585,484 Gasser May 18, 1926 1,586,233 Anschutz-Kaempfe May 25, 1926 1,840,104 Anschutz-Kaempfe Jan. 5, 1932 1,942,604 Kennedy Jan. 9, 1934 2,013,106 Nagel et al. Sept. 3, 1935 2,014,825 Watson Sept. 17, 1935 2,371,236 Gille et a1. Mar. 13, 1945 2,446,096 Moore July 27, 1948 2,446,325 Gille Aug. 3, 1948 FOREIGN PATENTS Number Country Date 439,227 Germany Jan. 12, 1927 516,185 Great Britain Dec. 27, 1939 

